Technical Field and Prior Art
The invention relates to a tuneable active microcavity and a process for manufacturing a tuneable active microcavity.
The invention is applicable to many applications including infrared detection and emission.
In general, a cavity is composed of two mirrors separated by a thickness e, regardless of the dimensions of the cavity. The thickness of a cavity satisfies the relation e=kxcex/2n where xcex is the resonant wave length, n is the refraction index of the medium between the mirrors and k is an integer. The resonant frequency of the cavity is made tuneable by moving the mirrors with respect to each other.
If the cavity is active, an active material such as CdHgTe is placed between the mirrors.
According to known art, a method of moving the mirrors in a cavity makes use of piezoelectric elements.
A tuneable cavity according to a first embodiment of prior art is described in FIG. 1. The cavity comprises 2 mirrors 1 and 2 and a piezoelectric element 3 included between the two mirrors. The distance between the two mirrors is modified by applying a voltage to the piezoelectric element 3.
The piezoelectric materials cannot be machined to be made very thin. The result is that the cavities obtained using the embodiment according to prior art described in FIG. 1 have a minimum thickness of the order of one millimeter. It is then no longer possible to make microcavities, in other words cavities with a thickness of the order of one micron.
A tuneable cavity according to a second embodiment of prior art is shown in FIG. 2.
According to this second embodiment, a piezoelectric actuator external to the cavity is used to vary the thickness of the cavity.
The tuneable cavity includes two mirrors 1 and 2, a piezoelectric actuator 4, a support arm 5 and a fixed support 6. The piezoelectric actuator 4 is located outside the cavity defined by mirrors 1 and 2.
A first mirror (mirror 1 in FIG. 2) is fixed to the support 6. The second mirror (mirror 2) is fixed to the piezoelectric actuator 4, which is fixed to a first end 5A of the support arm 5, the second end 5B being fixed to the support 6. The mirrors 1 and 2 are placed facing each other to form the cavity.
The mirror 2 can be moved with respect to mirror 1 under the action of a voltage applied to the piezoelectric actuator, thus inducing a change in the frequency of the cavity. This type of device has reference problems. It is not easy to precisely adjust the position of the mirror 2 with respect to the position of the mirror 1 located on the fixed support 6. The precision with which the mirror 2 can be positioned with respect to the mirror 1 is not better than approximately one millimeter. It is then only possible to make cavities with a minimum thickness of the order of one millimeter.
Therefore, it is also impossible to make tuneable microcavities according to this second embodiment.
However, tuneable microcavities are known elsewhere. One of the two mirrors in the microcavity is then placed on a deformable membrane. However, the membranes used are not very deformable. In this case, the tuneable frequency band is not very wide. For example, the tuneable band is typically 5% for a central wave length of 1 xcexcm.
The invention does not have the above disadvantages.
The invention relates to a tuneable active microcavity comprising a first mirror, a second mirror and a layer of active material between the two mirrors, characterized in that the tuneable microcavity comprises:
a piezoelectric actuator with a first face fixed to the first mirror,
a mechanical structure fixed to the second mirror,
mechanical connecting means, fixed firstly to a second face of the piezoelectric actuator such that the piezoelectric actuator is outside the space between the first and second mirrors, and secondly to the mechanical structure so as to put the first and second mirrors approximately parallel to each other and at a predetermined distance from each other,
each of the two mirrors being free to move under the action of a control signal applied to the piezoelectric actuator.
The invention also relates to a process for manufacturing a tuneable active microcavity including a first mirror, a second mirror and a layer of active material between the two mirrors, characterized in that it comprises the following steps:
fix the first mirror to a first face of a piezoelectric actuator so as to form a first assembly,
rigidly assemble an assembly composed of the second mirror and the layer of active material with a mechanical structure in order to create a second assembly,
move the first assembly and the second assembly with respect to each other such that the piezoelectric actuator is outside the space located between the mirrors and that the two mirrors are approximately parallel to each other and located on each side of the layer of active material,
fix the mechanical structure to a second face of the piezoelectric actuator using mechanical connecting means (14, 15), when the two mirrors are at a predetermined distance from each other.
One advantage of the invention is that it enables a very precise adjustment of the relative position of the mirrors that form the microcavity.
It is then possible to make a tuneable microcavity with a thickness of the order of a few microns, for example of the order of 2 to 5 xcexcm. This type of device can advantageously detect gases with absorption bands in the medium infrared (2-5 xcexcm). The wave length can be tuned over several hundred nanometers.